POWER May 2021 - 20

NUCLEAR POWER
Advanced Technology, Innovative
Solutions Enhance Nuclear
Operations and Maintenance
Incorporating new technology and solutions being used by others could help nuclear
power plant staff improve operations and maintenance practices.
Aaron Larson
uclear power plants face many challenges.
Monitoring heat exchangers
for signs of trouble, upgrading
pump bearings to minimize the likelihood
of failure, and improving human performance
are just a few examples of actions
that could enhance operations and maintenance
(O&M) at nuclear facilities.
N
Eddy Current Testing Evolves
One nuclear power plant component that
has required a lot of attention over the
years is the steam generator in pressurized
water reactor (PWR) units. Steam
generators separate the primary water
circuit from the secondary water circuit.
In PWRs, pressurized primary water,
which is radioactive, is pumped through
the reactor to remove the heat generated
by fission of uranium fuel in the core. The
heat is transferred to the non-radioactive
secondary water system through thousands
of tubes in the steam generator,
producing the steam used to drive turbine
generators to produce electricity.
Steam generators can be up to 70 feet
tall and have as many as 16,000 threequarter-inch-diameter
tubes. The problem
with many original designs was that the
tubes were not robust enough to withstand
the harsh conditions they encountered.
Most U.S. steam generators were
made with Alloy 600 tube material, a nickel-chromium-steel
alloy that proved to be
susceptible to thinning under some water
chemistry programs.
Tube denting also became a concern
over time. Denting is caused by corrosion
on carbon steel support plates in the
generators, and in the crevices between
tubes and the tube support plates. There
were also other causes for tube cracking,
leaks, and ruptures in PWR plants with
mill-annealed Alloy 600 tubes.
A common way to monitor steam
generator tube condition is through eddy
current testing. Eddy current technology
provides a fast and efficient solution for
20
detecting near-surface anomalies in conductive
material. A magnetic field from
a coil is induced into the tube material.
Anomalies in the material create changes
to the eddy currents that are sensed
by specialized instrumentation and then
evaluated by well-trained experts.
The technology was first developed in
the 1930s, but has evolved greatly since
that time. Zetec, a global provider of nondestructive
testing (NDT) technology, has
been supplying eddy current testing instruments,
probes, software and services for
more than 50 years. " The methodology
and the actual science, and the physics,
of course, hasn't changed, but the delivery
and analysis systems have improved tremendously, "
said Kevin Newell, manager
for Zetec's RevospECT Group. Newell noted
that the speed of eddy current operations
has increased well over tenfold since
he began working in the field in 1983.
Analyzing the data has also evolved
since Newell started. Certain tasks that
once required a trained technician can
now be done using computer programs.
" You basically set up a routine for the
computer to go through and look at all
these signatures, and you train it to recognize
what are normal signatures and
what are abnormal signatures, " Newell
said. The final analysis is confirmed by an
expert, but the data screening performed
by the automated system is invaluable.
" We're just embarking on adding now
some very focused machine learning
attributes, " said Newell. He noted that
computers will often identify " false positives, "
that is, anomalies that look like
potential issues, but aren't actually problems.
" That has to be sifted through, " he
said. " Machine learning helps eliminate
those false positives. "
Of course, all of the data is ultimately
important for trending purposes. " We
have literally decades of information now,
if a plant is still operating, where we have
that initial inspection, and we can see how
the tubes changed over time. And that's
www.powermag.com
where our program of historical data compare
comes into play, " said Newell.
Many of the original steam generators
have been replaced today. Most have
been upgraded to thermally treated Alloy
690 tube material, which is better at
resisting corrosion and has not demonstrated
the same propensity for cracking
as Alloy 600. Newell said that has caused
a shift in Zetec's focus.
" Now, all of a sudden, from the steam
generator perspective, those teams have
shrunk, because they're just not having
those problems as they did before, " he
said. These days, balance-of-plant components
have become more of an emphasis,
that is, feedwater heaters, lube oil
coolers, main and auxiliary condensers,
and other heat exchangers. " That focus
has shifted, and those are now pretty
large inspections, with a lot of personnel,
so that's where we've transitioned
our steam generator technology into the
balance-of-plant operations, " Newell said.
Better Reactor Coolant Pump Bearings
Reactor coolant pump bearings aren't
as prone to failure as steam generator
tubes, but when a station blackout (SBO)
occurs and pumps run down without
power, it can pose significant challenges
for these hydrodynamic bearings. Michell
Bearings, a UK-based bearing designer
and manufacturer, has recently completed
successful SBO tests on a thrust bearing
for a new-build nuclear plant. In what
the company claims is one of the world's
first successful bearing tests without the
use of any externally powered auxillary
systems, the innovative bearing was able
to survive the SBO condition.
" The challenge for us was to be able
to design the bearing in such a way that
it could safely run down from full speed
to rest whilst supporting quite challenging
loads, and then restart again without
sustaining damage in the process, " explained
Geoff Humble, sales and marketing
director with Michell Bearings.
POWER | May 2021
http://www.powermag.com
POWER May 2021

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